Reservoir Conditions Of Argillaceous Dolomite In Jianghan Basin

Conventional oil and gas is continually consumed,and demand for energe continues to rise,which forces the explorer to prospect new areas.Practice of North American exploration and development reveals that unconventional oil and gas play an increasingly important role in the energy structure.Shale oil and gas were the undoubtedly greatest potential of the unconventional oil and gas.In recent years,shale oil was also found in argillaceous dolostone reservoir of Jianghan Basin,Junggar Basin and Santanghu Basin,which widens the exploration range of shale oil.Research on the storage mechanism of oil in shale was already relatively mature,but there is still lack of study in the field of oil in argillaceous carbonate.Taking shale oil reservoirs of Jianghan Basin Xingouzui Group for example,thin section analysis,scanning electron microscopy,elemental analysis,cathodoluminescence,nano-CT and mercury intrusion and other experiments,were analyzed for argillaceous dolomite genesis mechanism,reservoir space characterization and controlling factors of reservoir property of Jianghan Basin Xingouzui Group.On the basis,storage mechanism of shale oil reservoirs was discussed.The results show: dolostone is composed of mud-sized crystalline dolomite,with mud crystal texture,subhedral-anhedral dolomite,characteristics of higher calcium content and lower degree of order,and the dolomite is red under cathodoluminescence,which indicate that the dolostone was deposited in the higher salinity anoxic reduction environment.Thus it may be inferred that the argillaceous dolostone is peneprimary dolostone formed directly by chemical precipitation.This study applied scanning electron microscopy to quantitatively characterize porosity of dissolved pore and primary pore(intergranular pores and intercrystal pores)respectively,and organic porosity is calculated by the material balance principle and nano-CT quantitatively.The results show that reservoir space of study area is mainly primary pore,and dissolved pore and organic pore are not development.Results of nano-CT three-dimensional reconstruction show better connectivity between pores,but there are relatively isolated pores present.Also we discussed the control factors of shale oil reservoir properties,such as lithology,mineral composition,diagenesis,the thermal evolution of organic matter and microscopic pore structure.The physical properties of argillaceous dolostone reservoir are the best of the study area,dolostone followed,and dolomitic mudstone is the worst.The correlation between porosity and mineral content is poor.Among them,the physical properties have a positive relationship with carbonate minerals,and the clay mineral content and permeability have a negative correlation.Dissolution plays a positive role on the physical propertiy of shale oil reservoir,but the effect is not obvious.Cementation and compaction have a significant damaging effect on reservoir.With the larger pore and throat and the worse sorting and the better connectivity,the physical properties of shale oil reservoir becomes better for the microscopic pore structure.